Magnetic testing device



Sept. .20? 1938,

E. 'FROBSE MAGNETIC TESTINE DEVICE Filed NOV. 22, 1934 my ein I Fg g, 2a

Figa.

Inventor:

Ernst Prob' '38, by Nam/ZV H i tt ornzy.

. Patentedv Sept. 20,- 1938 UNITED lsrlrrias PATENT OFFICE 2,130,882 MAGNETIC 'ras'rmc nevica Animation November zz, 1934, serial No. '154,232 In Germany January 8, 1934 4 claim. (ci 11s-iss) My invention relates to magnetic testing devices and methods and has forits principal object the location of defects in magnetic materials and the discovery of defective welds betweenmembers composed of magnetic materials.

It is an object of myinvention to provide an arrangement for detecting variations in the normal components of magnetic field or of leakage flux in the proximitybf a tested object.

Another object of my invention is to provide 4a testingarrangement which is independent of thel absolute values of leakage flux or stray ileld in the vicinity of the tested object.

Still another object is to--provide an arrangement for detecting variations in the rate of change offield-or ilux with respect to distance measured along' the test piece.

- Other 'and further objects and advantages will become apparent as the description proceeds. -0 In carrying out my invention'in its preferred form, an object or piece to be tested is magnetized preferably longitudinally and means are provided for detecting variations in the strength of the component of field normal to the surface of the tested object. The detecting device may consist of an electrical winding connected to a currentfresponsive device and having an eccentrically rotating or vibrating core'. The core may consist of a rotating cylinder having a segment thereof cut away so that, as the cylinderl rotates, the magnetic permeance of portions oi' the space in the electrical winding varies and. there is a tendency for voltages to be induced in the winding if the magnetic field due to leakage flux isnot uniform. I

In order to make the response independent of the absolute value of the leakage flux or the stray field, the segments cut from the cylindrical core may be symmetrical with respect to the axis of 40 the core or radially symmetrical so that the average value of the flux in various portions -of the core is constant if the rate of variation in neld. along the test piece is uniform. However, if ,the rate o1' variation in field is not uniform, voltage will be induced in thewinding for reasons to be explained. In this way, the device is made to respond only in case of defect and no response is obtained as a result of the normal field gradient along a homogeneous test piece. L t0 The features of my invention which I believe to be novel and patentablewillA be pointed out inA the claims appended hereto. A better understanding of .my invention may b e .obtained by referring to the following description taken in 55 connection 'with'. the laccompanying drawing 4in which Fig.- 1 represents schematically a flaw detector responsive to the absolute value oi.' leakage flux or stray field; Fig. 1a is a cross-section through a portionof the core of the detecting device shown in the arrangement of Fig. 1; Fig. 2 5 represents diagrammatically one embodiment of my invention in which the detectorl has a symmetrical coreto make the response independent of the absolute value of stray field; Fig. 2a repref sents a section of the core shown in Fig. 2 cut by 10 the plane a-a; Fig. 3a is a bottom view of the core shown in Fig. 2 with one position represented in full lines and another position represented in dotted lines; Fig. 3b is an elevation of the lower portionof the core in one position; Fig. 3c is an 15 elevation of the lower portion of the core in another position; Fig. 3d 'is a graph explaining. the principle of operation of the embodiment shown in Fig. 2; Fig. 4 illustrates a modified shape of magnetic core which may be used in the field detector; Fig. 4a represents a section cut .through the core of Fig. 4 by the plane a-a';

`magnetized preferably longitudinally in any desired manner as by means of permanent magnets or electromagnets. If desired, the field remaining after the piece II has been magnetizedmay beemployed for making the test. l

' The detecting device I2 consists of an electrical winding Il having a rotating core I4 of magnetiz- `able material with an eccentric portion I5 thereof cut away. If desired, the portion J5 may be 40 lled in with a non-magnetic material, such as brass, or preferably some material having substantially the same density as the core I4.

It will be evident that if the component oi.' magnetic field normal to the surface of the test piece II is not uniform along the test piece, the magnetic field within the winding I3 will not be uniform and more flux will thread the winding I3 when the core Il is in the position with the greater amountof iron in the portion oi' the 50 space within the winding Il in which the field strength is greater. Less flux will thread the winding I3 when the non-magnetic portion I5 oi.' the vcore Il is toward the side of the winding I3 Where the eld strength is greater. Consequently, rotation of the core I4 will produce variations in flux threading the winding I3 and a voltage will be induced inl the winding tl3. If there are no defects in the piece II, the iield gradient along it will be uniform and no change in voltage induced in winding I3 will be perceived as the detector I2 is moved along the piece I I. However, if there is a defect in piece II.. there will obviously be an abrupt change in the leakage ilux at the defect and consequently an abrupt change in the' component of magnetic field normal to the piece II at the defective point. This abrupt change will necessarily result in an abrupt variation in the voltage induced in the winding I3 which will be perceptible if suitable means, such as a telephone receiver, is used to indicate the voltage induced in the winding I3.

It is evident, however, that since the voltage induced in the winding I3 depends upon the degree magnetization of the piece II it will be necessary to distinguish between variations in induced voltage produced by variations in the amount of magnetization of thepiece II and those caused by defects therein. This may make the discovery of defects diicult, particularly if it is desired to magnetize the piece I I very strongly in order to increase the sensitivity of the device. In such cases I prefer to modify the shape of the rotating core I4 by making the portions cut away from the cylinder symmetrical .with its axis of rotation or with a plane through the axis of rotation. A suitable form of core is shown, for example, in Fig. 2 at I6. Referring to Fig. 2a, it will be seen that symmetrical segments I1 and 'I8 have been cut away from the lower end of the core i6. Any suitable means may be employed for rotating the core I 6 but in order to avoid electromagnetic eifects, I prefer to avoid the use of an electric motor. I may employ an air motor, for example, consisting of blades I9 cooperating with a :nozzle 29 which is supplied with compressed air by tubing 2i. The winding I3 may beconnected to a pair of headphones 22 through any suitable type of amplier 23 if desired, for the sake of increased sensitivity. l In the arrangement of Fig. 2, the test piece II may be strongly magnetized, if desired, by placing against it a magnetic yoke 24 having a current-conducting winding 25 supplied by a suitable source of current 26. Fig. 2 illustrates the application of the invention to detecting ilaws in a weld 2l in the piece I I.

The operation of the apparatus shown in Fig. -2 will more readily be understood by referring to Fig. 3 in which I have shown a portion of the core I6 in two different angular positions. In Fig. 3a is shown an end view; Fig. 3b is an elevation in one angular position represented by the full lines in the end view 3a; Fig. 3c is an elevation in another angular position represented by the dotted lines in view l3a and; Fig. 3d is a graph representing flux or eld strength plotted along the vertical axis against distance I along a'test piece plotted along a horizontal axis.

The curve 28 represents a uniform variation lin f field strength which might be produced 'in the vicinity of a test piece II without a flaw. `Varia-- \tn in stray field along the test piece is to be` in the piece. A defect in the test piece necessarily .75

introduces a vgreatincrease in reluctance in the test piece at the point where the defect occurs and, consequently, forces a greater amount of ilux outside of the test piece, adding to the leakage ux and therefore abruptly increasing the normal component of the stray eld.

'I'he point 39 represents the average value of the curve 28 across the width of the core I6 of the detector unit I2. It is apparent that the average value of the field acting in the two cut-away portions I'I and I8 of the core I6 in the position of Fig. 3b is the same as the average value of the curve 28 shown at point 30. This is also the average value of the field acting on the remaining portion I6' of the core I6. When the core I6 rotates to the position of Fig. 3c, yowing to the fact that the curve 28 is linear, the average value of the field acting in portions I1 and I8 of the core I6 as well as in the portion I6' is still equal to the value represented by the point 30 although the location of the portions I'I and I8 has changed. Consequently, there is no change in flux induced in the core I6 and no voltage is induced in the Winding I3 as the core I6 rotates.

If, however, the stray eld varies non-linearly as shown in the curve 29, the average value of ,the segments 3i--32 and 33-34 of the curve 29 corresponding to portions II and I 8 respectively of the core I6 in-the position of Fig. 3b will obviously not beequal to the average value of the curve 29 across the entire face of the core I6, or from point 3| to point 34. -When the core I6 is in the position of Fig. 3c the average eld acting .on portions I'I and I8 is of course the average curve 29 from point 3| to point 34. Consequently as the core I6 rotates from the position of Fig. 3b to th'e position of Fig. 3c the total i'lux threading the windings I3 will be changed and a voltage will be induced in winding I3 by rotation of the core I6. A sound will therefore be heard inthe phones 22 when the detector I2 is brought in proximity to a defect but no sound will be heard when the detector is adjacent `moved along will not affect the indications obtained. Likewise a constant rate of change of the stray field strength does not aiect the indications. 'Ihe rate of change of stray field along the test. piece may be designated by the mathe-A matical name derivative of stray eld with respect to distance measured along the test piece. Similarly the rate of variation in the rate of change is the derivative of the derivative or the 'secondderivative of stray eld with respect to distance measured along the test piece.

Although I have shown what I regard as a suitable form of .core in Fig. 2, it will be understood that I am not limited to this precise arrangement. My invention,` obviously, includes any arrangement of vibrating or rotating core material in which the magnetizable parts are maintained symmetrical with respect to an axis or a ixedor'a rotary plane through an axis. For example, I may use the form of core illustrated in Figs. 4 vand 4a. in which the lower end of the core tapers from a. cylindrical section to an elliptical sectlon. Another form of core which I have found suitable is illustrated in Figs. 5 and 5a in which sectors 3I are cut away from the lower portions of the core I6.

In accordance with the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider torepresent the best embodiment thereof but I desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a magnetic testing system including means for longitudinally magnetizing an object of magnetic material to be tested, a detecting device adapted to be placed in proximity to the tested object so as to be responsive to the component of' magnetic iield normal to the object, said detecting device comprising an electrical winding having a cylindrical core of magnetic material with portions thereof out away which are symmetrical with respect to the axis of the core, said core being mounted for rotation about its axis, and means responsive to current induced in said winding.

2. A magnetic testing device comprising an electrical winding, current responsive means connected thereto, and a core of magnetic material in the form of a cylinder with portions thereof cut away to form a radially symmetrical solid, said core being in inductive relation to said winding andl being adapted to be rotated about its axis in a magnetic eld to be examined.

3. A device for magnetically testing a test piece comprising an electrical Winding, current responsive means connected thereto, and a core for said winding mounted for rotation about its axis having portions of equal reluctance movable in their relative position with respect to a test piece, and a portion of different reluctance, the arrangement being such that the portions of equal reluctance remain symmetrical with respect to ille axis of the remaining portion of the core when such relative movement occurs.

4. A method of testing an object composed of magnetic material which comprises magnetizing the object, passing a current-conducting winding along the object, varying the relative reluctance of portions of the space within the coil in such a manner while rotating the core about its axis that the portions of varying reluctance remain symmetrical with regard to the axis of the core, and observing the induction of current in the coil.

ERNST FaoBsE. 

